From Fully Strained to Relaxed: Epitaxial Ferroelectric Al_1-xSc_xN for III-N Technology

The recent emergence of wurtzite-type nitride ferroelectrics such as Al1-xScxN has paved the way for the introduction of all-epitaxial, all-wurtzite-type ferroelectric III-N semiconductor heterostructures. This paper presents the first in-depth structural and electrical characterization of such an epitaxial heterostructure by investigating sputter deposited Al1-xScxN solid solutions with x between 0.19 and 0.28 grown over doped n-GaN. The results of detailed structural investigations on the strain state and the initial unit-cell polarity with the peculiarities observed in the ferroelectric response are correlated. Among these, a Sc-content dependent splitting of the ferroelectric displacement current into separate peaks, which can be correlated with the presence of multiple strain states in the Al1-xScxN films is discussed. Unlike in previously reported studies on ferroelectric Al1-xScxN, all films thicker than 30 nm grown on the metal (M)-polar GaN template feature an initial multidomain state. The results support that regions with opposed polarities in as-grown films do not result as a direct consequence of the in-plane strain distribution, but are rather mediated by the competition between M-polar epitaxial growth on an M-polar template and a deposition process that favors nitrogen (N)-polar growth

Modulation of Electrical Conductivity and Lattice Distortions in Burolk HVPE-Gwn GaN

The nature of self-organized three-dimensional structured architectures with spatially modulated electrical conductivity emerging in the process of hydride vapor phase epitaxial growth of single crystalline n-GaN wafers is revealed by photoelectrochemical etching. The amplitude of the carrier concentration modulation throughout the sample is derived from photoluminescence analysis and the localized heterogeneous piezoelectric response is demonstrated. The formation of such architectures is rationalized based on the generation of V-shaped pits and their subsequent overgrowth in variable direction. Detailed structure analysis with respect to X-ray diffraction and transmission electron microscopy gives striking evidence for inelastic strain to manifest in distortions of the P63mc wurtzite-type structure. The deviation from hexagonal symmetry by angular distortions of the β angle between the basal plane and c-axis is found to be of around 1°. It is concluded that the lattice distortions are generated by the misfit strains originating during crystal growth, which are slightly relaxed upon photoelectrochemical etching

Microstructure effects on the phase transition behavior of a prototypical quantum material

Materials with insulator metal transitions promise advanced functionalities for future information technology. Patterning on the microscale is key for miniaturized functional devices, but material properties may vary spatially across microstructures. Characterization of these miniaturized devices requires electronic structure probes with sufficient spatial resolution to understand the influence of structure size and shape on functional properties. The present study demonstrates the use of imaging soft X ray absorption spectroscopy with a spatial resolution better than 2 amp; 956;m to study the insulator metal transition in vanadium dioxide thin film microstructures. This novel technique reveals that the transition temperature for the conversion from insulating to metallic vanadium dioxide is lowered by 1.2 K 0.4 K close to the structure edges compared to the center. Facilitated strain release during the phase transition is discussed as origin of the observed behavior. The experimental approach enables a detailed understanding of how the electronic properties of quantum materials depend on their patterning at the micrometer scal

Ferroelectric BaTiO3 thin-film optical waveguide modulators

High-quality BaTiO3 epitaxial thin films on MgO substrates have been grown by pulsed-laser deposition. Both, c-axis and a-axis BaTiO3 orientations were studied. Mach-Zehnder optical waveguide modulators with a fork angle of 1.7degrees have been fabricated by ion-beam etching. The waveguides are of the ridge type, the BaTiO3 thickness is 1 mum, the ridge step 50 nm, and the width 2 mum. Light was coupled into the waveguides from optical fibers. The BaTiO3 waveguide propagation losses are 2-3 dB/cm. Electrodes of 3 mm length were deposited besides the waveguides. Electro-optic modulation has been demonstrated with V-pi=6.3 V at 632 nm wavelength and V-pi=9.5 V at 1550 nm wavelength for the a-axis samples, and with V-pi=8 V at 632 nm wavelength and V-pi=15 V at 1550 nm for the c-axis samples. Theoretical modelling of the Mach-Zehnder modulators for both crystalline orientations of the BaTiO3 films gave the Pockels coefficients r(51)=80 pm/V for the c-axis film and an effective Pockels coefficient r(eff)=734 pm/V for the a-axis films at 632 nm wavelength. (C) 2002 American Institute of Physics

Levodopa for treatment of parkinson’s disease: Study of 120 cases

Parkinson’s disease is a common adult onset neodegenerative disorder , induced mostly by the death of dopamine containing cells in the black substance (substantia nigra) . Hypothesis of a possible etiology of Parkinson’s disease may be complex interaction between environmental toxic factors, genetic and age (the incidence raises to almost 2% among over 65 year olds) related factors. We studied 120 patients with Parkinson’s disease living in Iasi, Romania, in 2006-2010, with ages between 50 and 80 years at baseline. Four patients were not included in the study as 2 of them being had neuroleptic medication induced parkinsonism and 2 of them have had ischemic strokes, thus the final lot of patients had 116 subjects.Based on clinical symptoms and different scales of assessment (such as UPDRS – Unified Parkinson Rating Scale, Hoehn and Yahr scale, the Global Disability and Quality of life scale) we evaluated the patient’s clinical symptoms before and during Levodopa treatment.The number of men in the study exceeded the women’s (90 men, 26 women, 8 rural patients and 118 urban patients). We have noticed a downfall in the clinical symptoms after Levodopa treatment